A gas turbine engine typically includes a compressor, at least one combustor, and a turbine. The air is pressurized in a compressor during operation and is then directed toward the combustor. The combustor commonly comprises multiple fuel nozzles to mix liquid or gas fuel with the compressed air efficiently before igniting the resulting mixture to create high temperature, high pressure combustion gases in a combustion liner of the combustor. The hot combustion gases are then directed via a duct towards a turbine, which is rotatably driven as the high temperature, high pressure combustion gases expand in passing over blades forming the turbine. The turbine produces work to generate engine thrust to propel an aircraft in flight, or to power a load in an electrical generator.
Since the operating pressure of the combustor is high, the combustion liner is contained within a case or pressure vessel. Fixed to this case is a fuel nozzle end cover that typically directs the flow of fuel from a fuel source to the fuel nozzles which injects the fuel into the combustion liner. Different fuels may be used for combustion depending on the type of performance and emission desired from the combustor. As a result, the end cover must be capable of preventing premixing of different fuel types and/or atomizing air within the end cover.
To meet the above requirement for a fuel nozzle end cover to deliver multiple fuel types separately, a flow insert is attached to and becomes part of the end cover using multiple braze joints therebetween. In a typical brazing process, the end cover and the flow inserts are first machined having very tight tolerance so that a diametrical gap therebetween is not more than a predefined number. For example, 0.005 inches may be the predefined number, but other dimensions are certainly possible. Braze wire, paste or foil, which is of an acceptable material for bonding the end cover and the flow inserts, is then taped or injected by syringe into the gap between the two components. Both components are then placed in a furnace, heated and cooled so that the insert bonds to the end cover to produce a joint capable of handling the temperature gradients and pressures applied to the end cover.
Although brazing provides the desired joint between the end cover and the flow insert, the process has its drawbacks. Depending on the configuration of the end cover, the resulting joints often cannot be inspected visually. However, detection of braze voids within a braze joint is significant to the detection and avoidance of flawed end covers. Furthermore, the braze joints are exposed to wide temperature ranges, for example between 0 and 800 degrees Fahrenheit, during operation of the gas turbine. This large temperature gradient in combination with high internal pressures, for example 250 lb/in2 or more, exerts increased stresses at the braze joints and potential failure of same. Braze joint failures could lead to leakage of fuel into hot gas cavities, and cause premature mixing and burning thereof within the end cover. The ensuing end cover failures could in turn result in catastrophic turbine failure, such as a forced outage. Thus, regular inspection of braze joint integrity is important during regular maintenance of the turbine. Accordingly, nondestructive examination procedures are required, such as pressure testing, X-ray, and ultrasonic inspection.
However, because the end cover is a thick plate, for example such enclosure often having a thickness of 4.5 inches or more, X-ray inspection is usually not possible for the end cover if the brazing filler material is a nickel alloy due to the low density of nickel. To better answer the challenges raised by the gas turbine industry to produce reliable and high-performance gas turbines, it is therefore desirable to find a brazing filler material which would provide the finished end cover with X-ray inspection capability. Further, a process to produce an end cover with X-ray inspection capability is desirable as well.